Far-infrared absorption in nitrogen gas

Abstract

We present a simple theory of the far-infrared collision-induced absorption spectrum of nitrogen gas. By assuming that the collisional pair dipole moment is generated through quadrupolar induction, and neglecting the small anisotropy in the N₂ intermolecular pair potential, we represent the spectral profile as the convolution of a single-molecule free rotation spectrum with a translational component, which is calculated using Mori theory. To close the Mori hierarchy we use information theory to estimate the ‘least biased’ form for the first-order memory function. In this way we avoid the need to make questionable assumptions about the system dynamics, and the problem reduces to the calculation of the first few moments of the translational spectrum, which is a relatively simple exercise in equilibrium statistical mechanics. The calculation of M ₀ ^tr, M ₂ ^tr, M ₄ ^tr and M ₆ ^tr is illustrated in an appendix.

Our results are in excellent agreement with experimental observations of the FIR spectrum of N₂ over a wide range of temperatures.